the Pool of Memory in Greek mythology
It is located in the underworld of Tartarus next to a white poplar tree. Its opposite is Lethe, The Pool of Forgetfulness. Souls are to drink from Lethe so they do not remember their lives on earth. Orpheus knew a secret password to get to Mnemosyne, though, which he passed on to a group of ghosts. They dropped the password to Hades's guards and were allowed to drimk from Mnemosyne. This gave them knowledge of past lives, as well as the future.